Adam Lanza: Who is the gunman behind the Connecticut school shooting?

Aminah Crawford, 4, joins in a vigil commemorating victims of a Connecticut elementary school shooting in Oakland, California, December 14, 2012. Twenty schoolchildren were slaughtered by a heavily armed gunman who opened fire at a suburban elementary school in Connecticut on Friday, ultimately killing at least 27 people including himself in the one of the worst mass shootings in U.S. history. The 20-year-old gunman, identified by law enforcement sources as Adam Lanza, fired what witnesses described as dozens of shots at Sandy Hook Elementary School in Newtown, Connecticut, which serves children from ages 5 to 10.

NEW HAVEN - The biggest outstanding question connected to the elementary school massacre in Newtown, Conn., is also the hardest to answer: Why?

Scant details have emerged about the 20-year-old man who gunned down 26 people - most of them children - at Sandy Hook Elementary School in the sleepy New England town Friday morning. After carrying out the deadly rampage, Adam Lanza shot and killed himself.

Police have not indicated whether a motive for the shooting, the second-deadliest of its kind in U.S. history. Reporters who have gone digging for details about shooter Adam Lanza have turned up with little so far.

A family member told investigators that he had "a form of autism," according to The Washington Post, which cited a law enforcement official.

Newtown High School classmates described him as "smart, introverted and nervous," someone who took extra care "not to attract attention," according to The New York Times. Other high school classmates called Lanza "generally a happy person" and "probably a genius," according to the Associated Press.

The Associated Press also reported that Lanza is believed to have suffered from a personality disorder, citing a law enforcement official apparently briefed on the investigation. Lanza lived with his mother, Nancy Lanza, whom he killed before driving her car to the elementary school Friday morning.

Lanza used two semiautomatic pistols to kill 20 children, many of them kindergartners. The weapons, along with a .223-caliber rifle that was found in the car, had belonged to Nancy Lanza, many news outlets reported.

Court records show Nancy Lanza and Peter Lanza, the shooter's father, filed for divorce in 2008.

As details of the shooting emerged Friday, there was confusion as to whether Lanza or his older brother, Ryan Lanza, was the suspect. Police questioned 24-year-old Ryan Lanza, of Hoboken, N.J., but a law enforcement official said he is not believed to have had a role in the killings, according to The Associated Press.

Robert Licata told the Associated Press that his 6-year-old son was in class when the gunman burst in and shot the teacher. "That's when my son grabbed a bunch of his friends and ran out the door," he said. "He was very brave. He waited for his friends."